This invention relates to piezoelectric knock sensors adapted for mounting on an internal combustion engine and particularly an interactive piezoelectric knock sensor of the type shown in U.S. Pat. No. 4,254,354 issued to John E. Keem. The knock sensor of this invention is an improvement over that shown in the above-identified U.S. patent in the mounting and electrical connection of its piezoelectric element to the flexing plate comprising the bottom of the case.
In an interactive piezoelectric knock sensor of the type contemplated, a flat piezoelectric disk is attached to a flexing plate which comprises the bottom of a sensor case having a rigid mounting stud affixed thereto. With the mounting stud attached to an internal combustion engine, the flexing plate serves as a spring between the mass of the remainder of the sensor case and the engine itself, which vibrate in response to engine knock induced vibrations interactively in a plurality of axial and transverse modes of vibration for a broader resonance frequency characteristic than is possible in the standard isolated resonance piezoelectric knock sensor. The piezoelectric disk, being attached to the flexing plate over a substantial portion of the flexing area thereof, partakes of these vibrations and generates electrical output signals in response thereto between its opposite faces. One face of the piezoelectric element is electrically connected to an output signal terminal while the other face must be reliably grounded to the flexing plate of the sensor case. In addition, the piezoelectric disk must be reliably affixed to the surface of the flexing plate so that it will indeed partake of the full range of vibrations thereof.
In the above-identified U.S. Pat. No. 4,254,354 it is disclosed that the piezoelectric element or disk may be affixed to the flexible plate with an epoxy material between the surfaces thereof and that, if the epoxy is not electrically conductive, electrical ground contact may be established by conducting elements pressed therebetween or by alternating projections and valleys in the surface of the flexing plate, the projections of which may contact the piezoelectric disk while the valleys are filled with the epoxy material. Both of these methods have been tried and found to be satisfactory for single samples of the sensor.
For mass production of the sensor in large quantities, however, it is desirable to provide a method of establishing the ground connection between the piezoelectric disk and the flexing plate which simplifies the assembly process by minimizing the number of parts involved and separate operations required and coordinates well with other steps in the assembly process.